John E. Kosin

RMI Company - U.S.A.

TREND IN PROCESSING, FABRICATION AND APPLICATION OF TITANIUM

1 - INTRODUCTION

Prior to discussion of the text of my presentation this

morning, I would like to review the availability and properties

of titanium (Fig. 1).

Titanium is not a rare element. In fact, you can see (Fig. 2)

that titanium is the fourth most abundant metal in the earth's

crust, following after aluminium, iron and magnesium.

Titanium is well dispersed throughout the earth and, as you can

see in Fig. 3, there are workable deposits of titanium ore

throughout the globe. Thus, it is not sensitive to geographical

or political problems that may exist in the world.

Fig. 4 shows the growth of the titanium industry by markets in

the United States from 1966 through 1985.

The free world consumption of titanium would be about twice the

numbers you see here. As you see, the defence usage has been

fairly constant over the period. Significant growth has

occurred in the commercial aerospace and industrial markets

since about 1976.

From Fig. 5 you can also see that the projected free world

capacity exceeds the demand. This is further illustrated

(Fig. 6) when one considers that only 5% of the TiO2

consumption goes into metal production.

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So we might ask, if titanium is so abundant, if it is so well

dispersed, why is it not used more?

The growth of titanium has been restricted to a certain extent

by the cost of the metal, cost of winning it from the ore. It

is fairly expensive in terms of energy. However, there has been

a gradual decrease in the amount of energy.

There has also been a rather large increase in the usage of

titanium.

So it seem reasonable to expect that the driving force for

changing trends in material use is to design around the key

properties of titanium, improve the performance and operating

costs, while at the same time, reducing the manufacturing

costs.

The key properties (Fig. 7) of titanium are fairly well known.

To review, they centre around two characteristics. First of

all, corrosion resistance. Titanium has excellent corrosion

resistance in all natural media, including chlorides and

organics, very resistant to salt water and chloride solutions.

The second main characteristic is that it has a low density,

about one-half that of iron and slightly higher than aluminium.

However the metal, as you can see, has high strength

capabilities. So, when you combine the low density with high

strength, you have a very efficient engineering material.

Another favourable characteristic of titanium includes its high

temperature strength.

Other interesting properties that, on occasion, are very useful

in design applications include:

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- it has very good ballistic resistance;

- it has low thermal expansion, which is very similar to

that of glass and composites, so it is very compatible

with those materials;

- it has a low elastic modulus, which makes it a unique

spring material, a very efficient spring material;

- it has, on the corrosion side, good heat transfer;

- it is very effective in heat exchangers.

Titanium also has some very unique electrochemical

characteristics, and it also has a very short radioactive

half-life, which can be useful in processing uranium and spent

uranium. I will expand on some examples of these properties

later in my presentation.

With this in mind, I would like to review the trends in

processing, fabrication and applications. Since time does not

permit me to cover all aspects, I will elaborate only on those

areas of major activity in the USA during the past three years.

2 - PROCESSING TRENDS

Responding to the aerospace, chemical process and electronic

industries needs for high-cleanliness, high purity titanium,

nonconsumable, cold-hearth melting is receiving a lot of

attention. The key is cold-hearth as opposed to drip-melting

directly into a cold-wall mold.

Production of these high performance engineering materials

requires highly controlled processing conditions. Both demands of

high purity processing environment and high performance

heating source can be met using:

- Electron Beam

- Plasma

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melting conditions. These technologies are similar with regard

to hearth melting.

Electron-beam melting (Fig. 8) is a vacuum process which

utilizes the thermal energy of the electron beam for melting

and refining operations. The heat in the prevailing vacuum

environment permits distillation of volatile elements including

vaporization of suboxides, evolution and removal of gases, and

vaporization of metal impurities with vapour pressures above

that of the titanium. Most notable in titanium is aluminium,

since it is common to most alloy systems.

Alec Mitchell of the University of British Columbia is working

on inline compositional analyses aimed at making compositional

adjustments during the melting process. Using the principles of

energy dispersion analysis (EDS), Mitchell has done pioneering

work in this area. The x-rays that generate the information are

excited by the very energetic beam which is used to heat the

surface of the liquid metal. Fig. 9 is an installation and

Fig. 10 is the molten pool and metal cascading over the hearth

into the mold.

Plasma-melting can be performed with a cold-hearth as well, but

in an inert gas atmosphere in the pressure range of 150 mbar

and 2 bar. Hence, one does not get the ultimate refinement as

with a vacuum system.

Electron-beam melting is the only process which simultaneously

can take advantage of the cold-hearth effect and the refining

effect of high and ultra-high vacuum.

Almost since its commercial inception, prompted by the

development of the gas turbine in the mid-forties, titanium has

been plagued by a defect (Fig. 11) known as:

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Type 1

LDI - low density inclusion

Hard Alpha

IRI - Interstitially Rich Inclusion

This defect is characterized by high hardness and higher

melting point, its composition is higher in nitrogen, may also

be higher in oxygen, and is usually depleted in major alloying

elements. Although it is denser (approximately 15%) than molten

titanium, it derives its low density connotation from the fact

that it is usually associated with a void. It acts as a crack

initiator and results in a premature fatigue failure.

Although an additional CEVAM has greatly reduced the incidence,

inspection techniques, namely ultrasonic, which has also

improved in the level of detection over the period, still

remains as the last chance of preventing a defect from getting

into the field.

Hence, the interest in hearth-melting, since defects that are

denser than the molten metal will sink to the bottom and become

entrapped in the skull which lines the hearth.

The procedure for removal of these denser materials has been coined

Defect-Free Titanium (DFT).

Commercially, hearth-melting is very attractive for

consolidation of scrap that otherwise would not be suitable by

CEVAM. In particular, small pieces and machining chips that are

generated with carbide tooling.

CEVAM (Fig. 12 and Fig. 13) begins with the fabrication of

electrodes (Fig. 14) made up of compressed sponge compacts

containing the alloy additions. Chemically compatible scrap can

become part of the electrode makeup, but they are rather large

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in size because they must be individually affixed to the

electrode. Smaller pieces can be included in the compact, but

such additions are limited by the compact strength. One can

also add limited quantities during the melting process by means

of a side-feeder.

With this type of scrap processed through a cold-hearth, any

segment of the electrode cross-section can be cast as dictated

by the mold-configuration. Hence, the attractiveness of a

combined nonconsumable hearth and CEVAM ingot.

This is reflected in the increased titanium scrap consumption

in ingot (Fig. 15) for the period 82-86.

Thus, one can take advantage of the refinement afforded through

cold-hearth melting and satisfy most industry specifications

with subsequent CEVAM melts since alloys shall be multiple

melted under vacuum or inert gas with the last melting cycle

under vacuum.

3 - FABRICATION TRENDS

In fabrication, I would like to discuss two areas, namely:

- Forging;

- Superplastic Forming and/or Diffusion Bonding.

Forging

I will discuss forging from two perspectives: 1) from what is

happening in the USA; 2) this combined with forging of

titanium.

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Thus, all my remarks relate to trends in the USA forging

industry as related to only titanium forgings. First I will

discuss the most popular titanium alloys used by the U.S.

forging industry; this will be followed by a brief description

of several developmental alloys. Second, I will discuss

developmental forging techniques related to attaining net shape

and improved properties. Illustrations n. 16 and 17 contain

information of general interest to forgers on current popular

titanium alloys. Included is the transformation temperature,

the forging range, the pressures required and the general

cracking resistance.

The alloys are classified in three primary groups: the alpha,

the alpha-beta and the new beta. As the beta increases the

cracking resistance improves, the transformation temperature

decreases, the forging temperature decreases, and the pressure

required decreases. As a result I suppose it is not surprising

that several near beta alloys have been used or considered for

forging applications. These include: 10V 2Fe 3Al (10-2-3)

transage alloys and the 38-6-44 or BetaC alloy. The importance

of this newer materials will be evident shortly when we discuss

the advances in the forging techniques (Fig. 18). Most of the

conventional forging methods can be applied to titanium and the

titanium alloys: forging, hammer forging, ring rolling,

mechanical or screw press forging. Because of the influence of

the die chilling on the deformation characteristics,

properties, structure and surface cracking, the conventional

forging process does have significant liabilities. This has

led to appreciable interest in technologies utilizing hot dies

which have an added benefit of achieving near or net shapes.

Let us now review two key differences in conventional, on the

left, versus net shape forging, on the right. These are:

- Structure/Property

- Product Yield/Cost

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Fig. 19 shows studies by Dr. Chen and compares the

macrostructures of Ti-6242Si alloy pancake made by the

different processes. As you can readily see, the macrostructure

becomes increasingly uniform as we progress from conventional

to warm die to isothermal forging.

Most commercial titanium alloys are forged near 1750° F (954°

C). As a result, isothermal forging - meaning die temperatures

near 1750° F (954° C) is extremely difficult since:

- Die materials like IN 100 are unrepairable

- Die heating systems are complicated

- Possibility of oxygen penetration into Ti

Hot die forging is conducted using normal metal temperatures

1750° F (954° C) and die temperature near 1400° F (750° C).

Advantages are:

- wrought alloy dies Astralloy, Waspalloy: lower cost and

weldable

- die heating simpler

- can produce near net and net shape

The impact of this technology is not only in improved structure

control, but in achieving shapes nearer the final

configuration.

Figure 20 illustrates metal utilization for various forging

processes for a typical titanium alloy such as Ti-6Al-4V.

Some examples of near and net shape forgings:

Figure 21: Ti-10-2-3 Near Net Instrument Housing

Figure 22: Ti-6Al-4V Near Net Disc

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Net shape (Figure 23). In this example, a transage alloy

pressure vessel dome was isothermally forged at 1500° F (800° C).

Top surface with bosses was forged net. Machining was required

only on the top of the bosses to get the desired flatness and

inside the dome which was a simple turning operation.

A cost comparison (Figure 24) of several processes by Kuhlman

of Alcoa shows the advantage of net shape forgings of beta-type

alloys.

This analysis was done on an engine mount and is based upon

production of 100 forgings. In summary, the machined part from

plate is 10% greater than a close-die part. However, the net

10-2-3 is 40% less expensive.

As you can see, new developments are being driven by reduced cost

through net shape forgings or improved performance through

property optimization.

4 - SUPERPLASTIC FORMING/DIFFUSION BONDING

Since superplastic forming and/or diffusion bonding offers

major savings in weight and manufacturing cost, there is

increased interest in the feasibility of applying the

techniques to the manufacture of a range of complex structural

aerospace components.

Diffusion Bonding is sometimes called diffusion welding. It is

a solid state welding process in which the surfaces are placed

in proximity under a moderate pressure at elevated temperature.

Coalescence occurs across the interface. Because diffusion

bonding requires heat, pressure and a vacuum, inert gas or a

reducing atmosphere, equipment is frequently custom built by

the user.

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Figure 25 is a wing carry through fabricated by diffusion

bonding 533 individual details.

Superplastic forming of titanium plate and sheet was also

developed as a reduced-cost method for processing of material.

From Figure 26, it can be seen that most of the titanium alloys

can be formed superplastically. The M-value or strain rate

sensitivity factor is a measure of superplasticity of the alloy

at a given strain rate. Any value greater than 0.5 is

considered a candidate for superplastic forming. In conjunction

with typical SPF elongation, it can be seen that Ti-6Al-4V is

the most desirable candidate. In Figure 27 one can see the

influence of grain size on flow stress. RMI has a grain size of

5µm. At this grain size, one can see how the flow stress and

M-factor are enhanced for superplastic forming. Figure 28 is

the typical microstructure of RMI sheet. Note that the

microstructures are similar before and after a simulated SPF

cycle.

Figure 29 is an example of the superplastic forming process.

These are examples of free blowing. That is, the final geometry

is not restricted or controlled by dies.

For the sphere, clean and treated sheets are edge-welded to

exclude air, but a pipe is attached for pressurization. After

bringing the unit to temperature, gas pressure, usually argon,

is applied until the desired shape has been achieved.

A combination of superplastic forming and diffusion (SPF/DB) is

being used on titanium to produce complex structures. Figure 30

is an example of the process. The sheets are prepared as with

SPF, except they are treated with stop offs where bonding is

not desired. Initial bonding occurs under pressure at

temperature and then internal pressure, through the pipe,

causes superplastic flow of the outer two sheets to fill the

die and create the configuration shown.

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Integrally stiffened structures are produced in this manner.

Following are examples (Figures 31, 32 and 33) of some

superplastically formed parts. Note the generally smaller bend

radii, deeper drawability and larger size that can be achieved

as compared to standard hot-formed (Figure 34) parts.

One U.S. aerospace company is superplastically forming a part

that previously consisted of seven individually hot formed

parts that were joined by conventional means, one can really

see the attractiveness of these fabrication methods for

producing hardware.

Spurred by the aerospace industry’s use of titanium discovery

of titanium’s excellent corrosion performance opened up a host

of unique applications for design engineering in the early

1960’s. Titanium corrosion resistance in neutral to oxidizing

environments is mainly due to a surface oxide layer. Engineers

in the chemical process industries, quick to exploit this

characteristic, were the first to apply titanium. At the same

time titanium unique electrochemical characteristics

revolutionized the chloro-alkali industry through the use of

precious metal coated titanium anodes. From these first two

industrial markets, titanium has expanded its non-aerospace

presence into more than 25 distinct market segments.

Figure 35 is the distribution and growth of those markets.

Its weaknesses include poor corrosion resistance to reducing

acids, red fuming, nitric acids and hot organic acids. If we

look at the general properties of titanium there are a few

observations that need to be made (Fig. 36). As one moves

towards the alpha alloys there is improved corrosion to

oxidizing environments at low to moderate strength levels. As

one moves to the beta alloys there is improved corrosion

resistance to reducing environments at higher strength levels.

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Alloyed versions including small amounts of palladium and

molybdenum increase the titanium resistance to reducing acids.

Additions of aluminum and vanadium increase the strength and

slightly reduce the range in the oxidizing region. But note the

beta alloy, the Beta C (Fig. 37), with the high attainable

strength up to 315° C and the range in both the oxidizing and

reducing sides. In the search for alternate energy sources, it

is not surprising that this alloy is rapidly gaining acceptance

in geothermal and sour gas wells.

5 - GEOTHERMAL

In geothermal, energy is extracted from the earth in the form

of high temperature hypersaline brines. Some of these brines

also contain H2S and CO2 at temperature ranges of 450° - 600° F

(232° - 315° C). These brines impose severe restrictions on

metals with limited corrosion resistance. In addition, high

strength and heat treatability are required due to the large

piping employed. In fact, beta titanium may be the only

economical way to produce energy using geothermal technology.

Figure 38 is an example of geothermal applications.

6 - SOUR GAS

Fossil fuel shortages have forced the oil and gas industries to

produce more corrosive deposits. These deposits typically contain

hydrogen sulphide (hence the term “sour”), pose an exceptional

challenge to the materials engineer.

Figure 39 shows the Beta CTM advantages in the oil field

market. These include high strength, corrosion resistance and

low density, which are all necessary for successful usage. In

addition, the low elastic modulus makes it nearly perfect for

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Figure 40, one can see the uses of Beta CTM for down-hole

applications.

Figure 41 and 42 are some of the casings and strings being used

in these installations. From their size and quantity, it is

easy to see the tremendous usage potential in these markets.

In summary, titanium usage will continue to grow based upon its

unique combination of properties aided by improved performance

and reduced total cost of the entire system.

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Fig. 1 - Titanium properties

Fig. 2 - Relative abundance of materials in the heth’s

crust

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Fig. 3 - Deposits of Titanium ores throughout

the globe

Fig. 4 - Growth of the titanium industry by markets

in the U.S. from 1966 through 1985

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Fig. 5 - Projected free world capacity and demand

Fig. 6 - Titanium ore consumption

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Fig. 7 - Titanium key properties

Fig. 8 - Electron beam processes

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Fig. 9 - Electron beam installation

Fig. 10 - Molten metal flow in the electron beam

cold earth

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Fig. 11 - Type I defect in titanium

Fig. 12 - Consumable electrode vacuum arc melting

(CEVAM) furnace diagram

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Fig. 13 - CEVAM furnace at RMI

Fig. 14 - Compressed sponge electrodes

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Fig. 15 - U.S. scrap consumption trend

Fig. 16 - Forging for titanium alloys

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Fig. 17 - Forging conditions for titanium alloys

Fig. 18 - Commercial techniques for forging titanium

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Fig. 19 - Macrostructure comparison in different

Ti-6242Si forgings

Fig. 20 - Metal utilisation for various forging

processes

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Fig. 21 - 10-2-3 near net shape instrument housing

Fig. 22 - Near net shape disc

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Fig. 23 - Transage alloy isothermal forging

Fig. 24 - Cost comparison in various fabrication

methods

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Fig. 25 - Wing carry through structure fabricated by

diffusion bonding

Fig. 26 - SPF characteristics for various Ti alloys

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Fig. 27 - Influence of grain size on flow stress

Fig. 28 - Typical microstructure of RMI sheet

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Fig. 29 - Examples of SPF processes

Fig. 30 - A SPF/DB process example

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Fig. 31 - SPF part

Fig. 32 - SPF part

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Fig. 33 - SPF part

Fig. 34 - Conventional hot formed parts

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Fig. 35 - Distribution and growth of titanium

Industrial markets

Fig. 36 - General properties of titanium alloys

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Fig. 37 - Elevated temperature yield strength vs.

Corrosion resistance for various titanium

Alloys

Fig. 38 - Geothermal applications of titanium alloys

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Fig. 39 - Beta C advantages in the oil field market

Fig. 40 - Examples of downhole applications for

Beta C alloy

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Fig. 41 - Beta C downhole piping

Fig. 42 - Beta C downhole piping

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